In the 1940's, Babcock & Wilcox (B&W) developed a cyclone furnace, which uses ash slagging, to burn low-grade coals. At that time, low-grade coals were considered unsuitable for pulverized coal combustion. The cyclone furnace concept was originally designed to 1) lower fuel preparation capital and operating costs by using relatively larger crushed coal particles (1/4" or less); 2) combust coal completely or nearly completely in a relatively small cylindrical chamber; 3) lessen flyash and convection pass fouling (as only 15 to 30% of the convecting fuel ash passes instead of 80% for pulverized coal firing) by melting the ash contained in the coal and separating a large percentage of it from the flue gas; and 4) accurately measure and control coal flow and air flow to each burner.
Cyclone furnaces fire relatively large crushed coal particles, approximately 95-97% passing through a 4 mesh screen. Many of these particles are much too large to burn completely in air suspension. To completely combust them, they are thrown against the inner wall of the combustion chamber, where they are captured by a molten slag layer. The combustion air passes over the incompletely burned particles (air scrubbing) stuck in the molten sticky slag layer, which captures and holds the heavier particles. While the large particles are trapped in the slag layer, the fine coal particles burn in suspension, which provides the necessary intense radiant heat supplied to the slag layer. Ideally, it is desirable to trap all large coal particles in the molten slag so that they can completely combust, leaving behind only ash to replenish the slag layer. Compared to a pulverized coal furnace, a cyclone furnace requires a relatively smaller combustion chamber.
Three types of burners have been developed for use with the cyclone furnace: scroll, vortex, and radial. With each of these burners, cyclone furnaces use a horizontally oriented cylindrical barrel (of water-cooled tube construction), typically 6 to 10 ft in diameter, attached to the front and/or rear of a boiler (main) furnace. A cyclone burner is positioned at the frontwall (upstream wall) of the cylindrical barrel, collinearly aligned therewith. Crushed coal and air (primary and tertiary (for scroll and radial burners only)) enter through the cyclone burner where the fuel is ignited. The coal-air fuel mixture is propelled to the combustion chamber where the larger coal particles are captured in the molten slag while the finer particles are burned in suspension in the combustion chamber. Main combustion (secondary) air is introduced into the combustion chamber to impart a swirl to the coal particles. Combusted products leave the cyclone furnace through the re-entrant throat (exhaust outlet). A molten slag layer develops and coats the inner surface of the combustion chamber. The slag drains to the bottom of the cyclone and is discharged through the slag tap. To capture the molten slag, the inside of the combustion chamber is provided with densely populated short pin studs that extend radially inwardly from its inner surface. A refractory lining material (insulation) is embedded in the pin studs to maintain the combustion chamber at a sufficient temperature to permit slag tapping from the bottom of the combustion chamber and significantly reduce the potential for corrosion.
In scroll and radial burner types, crushed coal is introduced tangentially to the inner wall of the cyclone burner to impart a swirl to the crushed coal. A vortex burner, on the other hand, introduces crushed coal from the burner end wall. Primary air is also introduced tangentially into the burner to further impart a swirl to the crushed coal in vortex and radial burners. In the scroll burner, primary air and crushed coal are premixed before they are introduced into the burner. The scroll and radial burners also use tertiary air to control axial flame displacement and to prevent coal from continuously recirculating at the burner end wall. In all three types, secondary (main combustion) air is introduced tangentially to the cyclone barrel in the same rotation direction as the coal swirling motion imparted by primary air to further impart a swirl to the coal. Primary air, secondary air, and tertiary air are typically tapped from a windbox (air duct), which supplies preheated air. The preheated air in the windbox is obtained by passing ambient air through a heat exchanger, which derives heat from the gases exhausting from the boiler to which the cyclone furnace is attached. Typically, the heat exchanger heats air to about between 550-600.degree. F.
The purpose of cyclone primary air is to distribute coal into the combustion chamber. Primary air enters the burner tangential to the cylindrical inner wall to impart a swirl to the crushed coal introduced into the burner or carry the crushed coal (in scroll burner) into the cyclone burner with a swirl. Primary air controls the coal distribution within the combustion chamber. Generally, primary air is about 10-20% of combustion air flow.
Secondary (main combustion) air is injected tangentially to create a swirling motion in the combustion chamber in the same direction as the swirl imparted by primary air. The swirling motion throws the large coal particles against the inside surface of the combustion chamber, where they are trapped in the slag layer and burn to completion. Secondary air is about 85% of the combustion air supplied to each cyclone.
Tertiary air enters the center of the burner along the cyclone axis, directly into the cyclone vortex. The purpose of cyclone tertiary air (for radial and scroll type cyclones) is to minimize coal recirculation at the "eye" of the burner. Generally, tertiary air is about 3% of combustion air flow.
Early cyclone furnaces were of the scroll type, which combines primary air and coal before entering the burner. Tertiary air is admitted at the center of the burner to minimize coal recirculation at the eye of the burner. In vortex cyclone furnaces, primary air is injected into the cyclone burner tangentially as with the scroll type, but the coal is introduced at the burner center. This configuration eliminated the tertiary air requirement. The radial burner concept was developed in the 1960s to solve the wear-block erosion problem. Like the vortex burner, coal and primary air are separately introduced into the radial burner. Coal is introduced tangentially in the same rotation as the primary air. The coal particles form a long rope (concentrated stream of coal) as it sweeps across the burner wear blocks and enter the cyclone barrel. This approach greatly reduced the concentration of coal recirculating around the burner, effectively reducing wear-block erosion. Tertiary air was introduced using the same axial entry location as with the scroll burner.
Modern cyclone furnaces incorporate radial burners to combust bituminous and sub-bituminous coals and scroll burners to combust lignite coal. The scroll burner is used to combust lignite coal because, with lignite firing, the primary air is mixed with coal during coal preparation, before introducing coal into the burner. When mixed with coal, primary air can heat coal to about 150-250.degree. F. (the mixture temperature). Due to evaporation of moisture in the coal, the temperature stays in this range even if the primary air temperature is raised, e.g., to 700.degree. F.
When operating properly, cyclone furnaces generate much greater heat than the water-cooled walls can absorb. The combined high heat release and low heat absorption rates ensure the high temperatures needed to nearly complete the combustion within the cyclone furnace and maintain the slag layer in a molten state. The intense radiant heat and high temperatures melt the ash into a liquid slag coating, which covers the entire cyclone interior surface except for the area immediately in front of the secondary air inlet. The refractory lining further assists this molten condition by limiting heat absorption to the water-cooled walls. The slag flows constantly from the cyclone into the main furnace where it drains through a floor tap opening into a water-filled tank.
Fuel suitability for cyclone furnaces depends on many characteristics of the fuel, such as heating value, ash content, moisture content, ash fusion temperatures, and viscosity of the fuel ash at the cyclone operating temperature. The most important consideration in cyclone firing is that the temperature in the cyclone furnace must be high enough to maintain a molten slag coating and cause the ash to flow continuously from the cyclone furnace.
This consideration was easily met for a wide spectrum of bituminous coals in the U.S. Currently, a significant portion of electric power generation in the U.S. is by units having cyclone furnaces.
To meet SO.sub.x emission requirement and to minimize fuel cost, it is highly desirable to burn sub-bituminous coal, which typically has a very low sulfur content and is relatively inexpensive. As a result, a large number of cyclone furnace units, designed to burn bituminous coal have been converted to burn sub-bituminous coal or a blend of mostly sub-bituminous coal. Sub-bituminous coal, however, is difficult to burn satisfactorily in most cyclone burners designed to burn bituminous coal because, due to the higher moisture content of the sub-bituminous coal, the cyclone temperature often is too low for the ash to continuously flow from the cyclone furnace. The cyclone furnace temperature is lower when burning sub-bituminous coal because of the necessity to evaporate the high moisture content of sub-bituminous coal and because the high moisture content delays combustion and thus reduces the percentage of combustion that occurs within the cyclone furnace. As a consequence, most cyclone furnace boilers that have been switched to sub-bituminous coal have to burn a blend of sub-bituminous coal with an expensive high heat value "kicker" coal or accept lower boiler efficiency due to increased discharge of unburned coal. The industry thus has been searching for economical ways to enable burning sub-bituminous coals, without blending them with more expensive bituminous or higher heating value "kicker" coals, as much economical advantage can be gained therefrom.
Thus, there is a need for a more economical way to burn higher percentages of low-grade fuel in cyclone furnaces. The present inventor has a found a more economical way to burn low-grade solid fuel, particularly sub-bituminous and lignite coals in cyclone furnaces.
The above background information is derived from my personal experience and observations, and from Babock & Wilcox's publication, STEAM, 40th Edition, Chapter 14, Cyclones, the disclosure of which is incorporated herein by reference as a general background information.